1. Field of the invention
The present invention relates to an optical information medium, on which can be recorded information which is optically reproducible, and in particular relates to an optical information medium having a sector information detector means suitable for a high density recording medium on which the information can be recorded and reproduced with a short wavelength of from 630 nm to 670 nm.
2. Description of Related Art
With the recent advances in development and application of a short wavelength laser, standardization has proceeded on a standard for DVDs (Digital Versatile Discs) which enables recording and reproducing at a high density, as well as the practical application thereof. On the DVD is provided a data area at least on one surface thereof, in which pits are formed as a means for recorded information, and a reflection layer made of metal film is formed on the data area.
For the DVDs mentioned above, a different standard for high density is determined, compared to the standard for the optical information medium being most general or popular now, such as a CD (Compact Disc). For example, with the optical pickup, it is determined to use a short wavelength of from 630 nm to 670 nm, and to use an objective lens of a high numeral aperture, such as of 0.6 in NA, etc. Accompanying this, for dealing with a tilt of the disc, the thickness thereof is determined to be 0.6 mm, about half (xc2xd) of that of the CD. However, for maintaining compatibility or interchangability in sizes between the CD having thickness of 1.2 mm, the disc of the DVDs have such a structure that two pieces of discs are filled up and pasted together. According to the DVD standard, it is standardized that one piece of the disc has a recording capacity of 4.7 GB at the maximum, in average thereof, i.e., video and audio information for a maximum 133 minutes long.
In the standardization process of the DVD standard, in the similar manner of the relationship between the CD-ROM and CD-R or CD-RW, at the beginning were assumed or expected to be standardized both an optical information medium for use or reproduction exclusively (for read-only) and an optical information medium which is recordable. However, the standardization at the beginning was made only on a DVD Video for video and on a DVD for reproduction only, such as a DVD-ROM which is targeted for recording computer data, but the standardization on the recordable DVD was postponed.
Heretofore, on the recordable CD, such as the CD-R, an address information called xe2x80x9cATIP (Absolute Time in Pregroove)xe2x80x9d is obtained through FM modulation of wobbles of a tracking guide which is formed in spiral and groove-like shapes. In contrast to this, on the recordable DVD, such as the DVD-R, in place of the ATIP, sector information, including the address information indicative of the position on the optical information medium, can be obtained through land pre pits which are provided in advance on land portions defined between the tracking guides together with the wobbles.
Such land pre pits, which are applied into the high density recording medium as mentioned above, are readout by means of an optical pickup together with the pits of the recorded signals, however, since the optical pickup is guided along the tracking guide by a tracking servo system under a normal condition, the land pre pits and the pits formed on the groove for recording data signals are separately readout by discriminating them to each other. However, depending upon the shape of the land pre pits, there is a possibility that the land pre pits are erroneously readout by being mistaken for the pits for recording of the data signals.
Then, it comes to be a cause of occurring errors in the readout of the data signals. In contrast to this, there is also a possibility that the land pre pits cannot be readout correctly depending upon the shape of the land pre pits, then it causes generation of errors in the sector information including the address and so on, thereby making the reproduction operation unstable.
Accordingly, an object is, in accordance with the present invention, for solving the drawback(s) mentioned above, i.e., the problem in the readout of the land pre pits for picking up the sector information on the optical information medium corresponding to the high density recording mentioned above, to provide an optical information medium, with which the data signals and the sector information can be readout with discrimination between them correctly, by removing the errors in the readout of the land pre pits indicative of the sector information and of the pits indicative of the data signals on the tracking guide when reading the high density signals optically.
For achieving the object mentioned above, according to the present invention, an optically clear difference is given between the land pre pits 6 indicative of the sector information provided on the land 8 and the pits indicative of the data signals provided on the tracking guide 3, thereby preventing the erroneous reading of mixing them up with each other. In more detail, the difference is in the lengths of the pits, and in the optical path length of a reproducing light beam incident upon from the optical pickup.
Namely, according to the present invention, there is provided an optical information medium, comprising:
a transparent substrate 1 for a reproducing laser beam to penetrate therethrough;
a recording layer 12 formed on said transparent substrate; and
a reflective layer 13 for reflecting the reproducing laser beam thereupon, thereby enabling the recording of the optically readable signals by means of a recording laser beam incident upon said transparent substrate 1. This optical information medium has a spiral-like tracking guide 3 on a side surface of said transparent substrate 1, on which said recording layer 12 is formed, and on lands 8 defined between the spiral-like tracking guide 3 are formed land pre pits 6 indicative of the sector information, including the address and so on, wherein the length of the land pre pits 6 is selected to be different from the lengths of the data pits.
On such an optical information medium, in which the length of the land pre pits 6 is different from that of the data pits, the signal obtained from the data pits formed on the tracking guide 3 can be discriminated from the signals obtained from the land pre pits 6, with ease. With this, there is no chance to readout the pits for recording the data signals, erroneously, in place of the pits for recording the sector information, thereby enabling the reduction of the reading errors of the data signals as well as the reading errors of the sector information.
For example, in the DVDs, the data pits lengths were from approximately 0.4 xcexcm to approximately 1.9 xcexcm. Accordingly, the length of the land pre pits 6 is selected to be shorter than 0.4 xcexcm, or alternatively to be the length exceeding 1.9 xcexcm. However, it is unrealistic to make the length of the land pre pits exceed 1.9 xcexcm, therefore it is preferable to select it to be shorter than 0.4 xcexcm.
In this instance, it is preferable to shorten the length of the land pre pits 6 as much as possible up to the length of the minimum data pit. However, if the length of the land pre pits 6 is too small in length, it becomes difficult to readout the sector information from the land pre pits 6. On the other hand, when the length of the land pre pits 6 becomes near the length of the minimum data pits, discrimination cannot be performed easily between the land pre pits 6 and the data pits by the pit length. Then, the length of the land pre pits 6 is selected to be from 50% to 83% of the length of the minimum data pit. For example, the length of the minimum data pits is 3 times as long as a channel clock in the DVD, therefore, the length of the land pre pits 6 is selected to be from 1.5 to 2.5 times the length of the channel clock.
Further, in the optical information medium according to the present invention, the optical path length for the reproduction laser beam from the incident surface thereof upon the transparent substrate 1 up to the reflection layer in the area of the land pre pits 6 is selected to be different from the optical path length for the reproduction laser beam from the incident surface thereof upon the transparent substrate 1 up to the reflection layer in the area of the tracking guide 3. In more detail, the film thickness of the recording layer 12 in the area of the land pre pits 6 is selected to be different from the film thickness of the recording layer 12 in the area of the tracking guide 3.
Further in more detail, an absolute value of the difference, between the film thickness of the recording layer 12 in the area of the land pre pits 6 and the film thickness of the recording layer 12 in the area of the tracking guide 3, is selected to be from xcex/200 to xcex/12. Namely, an effectively adjustable range in the length of the optical path length is from approximately 10 nm, for example, when the wavelength of the laser beam is 635 nm. Here, in particular in the DVD-R, since dye is used as the recording layer, the effect of an optical constant n of the recording layer gives influence upon the length of the optical path. For instance, the optical constant n of the dye layer is 3.0, it is enough for the film thickness of the recording layer to be 10.0/3.0=3.333 nm, and this can be converted to 635/3.333 =200 in the wavelength of 635 nm, i.e., it is apparent the film thickness has an effective function for adjusting the length of the optical path if it has a difference equal or greater than xcex/200.
Further, in a case of detecting the land pre pit signals by a single beam in accordance with a push-pull method, the adjustable range is within xcex/8. However, also of the effect of the optical constant n of the recording layer made of the dye layer, the film thickness of the actual recording layer comes to be a value smaller than that. Namely, in a case where the optical constant n of the dye layer is 1.6, xcex/8 is (xcex/8)/1.6=xcex/12.8, i.e., it is apparent that the adjustment is possible in the length of the optical path all over the range if it has xcex/12 substantially in the film thickness thereof.
In such an optical information medium, since the length of the optical path for the reproducing laser beam from the incident surface of the transparent substrate 1 differs between the areas where the land pre pits 6 and the tracking guide 3 are formed, respectively, and since it is possible to obtain a signal amplitude larger than that due to the physical difference of the land pre pits 6 formed on the transparent substrate 1, it is possible to detect the land pre pits 6 with ease.
Since no land exists only in that portion, i.e., in the portion of the land pre pits 6, substantially it is in such a condition that the tracking guide 3 is widen in width in a radial direction thereof. Therefore, when spin coating the dye material for forming the recording layer 12 on the transparent substrate 1, the leveling of the film thickness of the recording layer 12 is changed in the area of the land pre pits 6, and in particular, the film thickness Tg is changed compared to other portions of the tracking guide 3.
Also, the land pre pits 6 are formed in the transparent substrate 1 of the optical disk by a molding process and in that instance the protruding rims 7 are formed between the land pre pits 6 and the tracking guide 3 neighboring to both sides thereof, being lower than the depth of the tracking guide 3. This rim 7 gives influences upon the leveling of the recording layer 12 in the area of the land pre pits 6, in particular, when spin coating the dye material for forming the recording layer 12 on the transparent substrate 1, and it also gives influences on differences in the film thickness. Namely, if there exists the protruding rim 7 mentioned above, a resistance is changed in fluidity of the material for forming the recording layer, thereby changing the film thickness of the recording layer 12 formed. Then, by using this rim 7 in a positive manner, it is possible to make the film thickness different between the recording layer 12 in the area of the land pre pit 6 and the recording layer 12 in the area of the tracking guide 3.
However, it is not necessary for this rim to exist in a middle portion of the land pre pit 6 in a radial direction of the optical information medium. Namely, the rim 7 can give the effects even when it is interrupted on the way.
According to the present invention, errors in reading the data signal and the sector information are diminished, therefore they can be readout discriminatorily with certainty, when optically recording and reproducing the signals onto and from the optical information medium by using the optical pickup corresponding to the high density recording. Therefore, it is possible not only to record but also to readout the data signals with stability and certainty as well.